1) Field of the Invention
The present invention relates to a lenticular lens sheet for use in a rear-projection screen.
2) Description of the Related Art
So far, as a technique of displaying large-screen pictures, there has been known a method in which an optical image is enlarged and projected from a CRT or a liquid crystal panel through a projection lens onto a rear-projection screen. FIG. 4 schematically shows a construction of a common rear-projection television for image formation based upon this method, FIG. 5 is an illustration available for explanation about light beams, and FIG. 6 is a perspective view showing a screen to be employed for the FIG. 4 rear-projection television.
In the rear-projection television shown in these illustrations, optical images emitted from CRTs 1 respectively corresponding to R (red), G (green) and B (blue) are enlarged through projection lenses 2 and, after being reflected on a reflecting mirror 7, formed as an image on a rear-projection screen 5 comprising a Fresnel lens sheet 3 and a lenticular lens sheet 4. In this case, the Fresnel lens sheet 3 fulfills a function to substantially direct the incident light toward the position of a viewer, while the lenticular lens sheet 4 has a function to disperse the light emerging from the Fresnel lens sheet 3, at given angles in the horizontal and vertical directions in an appropriate proportion of distribution to expand a visible angle to an angle of a given scope.
On the other hand, in such a display system, due to convergent angles .epsilon. of the light beams projected from the respective R, G and B CRTs 1, the images on the screen 5 vary in color tones when the screen viewing position changes in the horizontal directions (see FIG. 5). That is, in the common disposition of the CRTs 1, the green CRT 1 is at the central position whereas the red and blue CRTs 1 take the positions shifted therefrom in the left- and right-hand directions, and hence, if changing the viewing position in the left- and right-hand directions, the red and blue increase in color level. This phenomenon is referred to as a color shift.
For this reason, for the purpose of decreasing the color shift, as a lenticular lens sheet devised so far, there has been known a double-sided lenticular lens sheet generally designated at numeral 10 in FIG. 7 in which an incident side (entrance) lenticular lens 11 formed by arranging a plurality of cylindrical lenses in parallel is placed on a light-incident side surface, while an emergent side (exit) lenticular lenses 12 formed by arranging a plurality of cylindrical lenses in parallel is located on an emergent side surface, and further, light-absorbing layers 13 are formed on non-convergent portions of the incident side lenticular lens 11 existing on the emergent side surface.
Furthermore, as the current lenticular lens sheet manufacturing method, a thermoplastic resin extrusion forming method has been taken because of a high productivity and others. According to this method, for allowing smooth extrusion throughout the overall width of a die, the temperature of the end portions of the die is adjusted to be higher by 5 to 10.degree. C. than that of the central portion thereof. This is for the purpose of compensating for the drop of the pressure at the end portions within a manifold by the decrease in viscosity.
However, since the thickness of the lenticular lens sheet is generally as thin as 0.5 mm to 1.5 mm, when a molten resin is put between shaping rolls in the prior extrusion forming method, the pressing working in the width directions of the shaping rolls becomes unequal so that the shaping rolls are largely distorted at their end portions with respect to their central portions, that is the so-called roll distortion takes place. For this reason, regardless of an intention of forming a double-sided lenticular lens sheet with a uniform thickness in the width directions, in fact, as shown in FIG. 8, an inter-lens distance t.sub.1 of the end portion of the double-sided lenticular lens sheet 10 (in more detail, the inter-lens distance t.sub.1 at an end portion of an viewable area of a rear-projection screen using a lenticular lens sheet) becomes thinner by a value exceeding several % than the thickness to at the central portion. In addition, due to the unexpected reduction of the inter-lens distance at the end portion, as shown in the same illustration, an incidence angle .theta..sub.1 of a light beam L incident on the emergent side lenticular lens 12 at the end portion of the double-sided lenticular lens sheet 10 becomes extremely larger than an incidence angle .theta..sub.0 of the light beam L incident on the emergent side lenticular lens 12 at the central portion of the double-sided lenticular lens sheet 10, which causes the overall light beam transmittance to go down.
Furthermore, in the case that, in the central portion of the double-sided lenticular lens sheet 10, the sheet thickness is set so that the light incident on the incident side lenticular lens 11 is condensed on the emergent side lenticular lens 12 surface and the light-absorbing layers 13 are provided on the non-convergent portions of the incident side lenticular leans 11 on the emergent side lenticular lens 12 surface, if the incidence angle .theta..sub.1 of the light beam L incident on the emergent side lenticular lens 12 at the end portion of the double-sided lenticular lens sheet 10 increases as mentioned above, a problem arises in that the exit of the light beam L from the emergent side lenticular lens 12 is blocked by the light-absorbing layers 13, that is, the so-called light eclipse occurs. Moreover, since the difference between the incidence angle .theta..sub.1R made when the light beam from the red CRT is incident on the emergent side lenticular lens 12 and the incidence angle .theta..sub.1B made when the light beam from the blue CRT is incident on the emergent side lenticular lens 12 becomes large, a large color shift takes place.
Incidentally, these problems originating from the reduction of the inter-lens distance do not take place in the case of a single-sided lenticular lens sheet 10x shown in FIG. 9 (that is, a lenticular lens sheet in which an incident side lenticular lens 11 is provided on an incident side surface for the light beam L and a flat emergent surface is formed at a portion from which the light incident on the incident side lenticular lens 11 exits). In the single-sided lenticular lens sheet 10x, the lenticular lens sheet thickness equivalent to the aforesaid inter-lens distance of the double-sided lenticular lens sheet 10 decreases from t.sub.0 to t.sub.1, and even if the light beam L emergent side surface changes from the emergent side surface S.sub.0 at the original position to the emergent side surface S.sub.1 at the position indicated by a dotted line, the relationship between the incidence angle .theta..sub.0 made with respect to the emergent side surface S.sub.0 when taking the original sheet thickness and the incidence angle .theta..sub.1 made with respect to the emergent side surface S.sub.1 after the reduction of the sheet thickness is .theta..sub.0 =.theta..sub.1, that is, no change takes place. Accordingly, the transmittance of the light beam L does not vary in the emergent side surfaces S.sub.0, S.sub.1. Further, although the color shift in the single-sided lenticular lens sheet becomes originally larger as compared with the double-sided lenticular lens sheet, the degree of the color shift does not vary due to the reduction of the sheet thickness.